1. Field of the Invention
The present invention relates to a golf club. More particularly, the present invention relates to a hollow head of a golf club of a wood type.
2. Description of the Related Art
For a golf club of a wood type, a head formed of a persimmon has been mainly used. In some days, a head formed of a carbon fiber reinforced resin spread. In recent years, a head formed by a metal material such as stainless steel, an aluminum alloy or a titanium alloy has become mainstream. In particular, the titanium alloy having a high specific strength has been used willingly. In order to reduce weight, a hollow structure is employed for a head of a wood type which is formed of metal.
The swing form of an average golf player is unstable. Due to the disorder of the swing form, the hitting point (a contact point in a face with a golf ball) is apt to be shifted from a sweet spot. The shift of the hitting point causes a bad shot. In respect of the suppression of the bad shot, a head having a large volume has been proposed and employed. Referring to a large-sized head, an effort has been made to reduce the thickness of each member constituting the head in order to decrease the weight.
The face of the golf club head comprises a loft. By the loft, a golf ball hit by the golf club is launched obliquely and upward and flies with a backspin. A launch angle obtained immediately after hitting and a backspin rate are important elements that influence the trajectory of the golf ball. An appropriate trajectory height is obtained by a proper launch angle and backspin rate.
In the golf club of the wood type, importance is attached to a flight distance. In respect of the flight distance, the skilled in the art have recognized that a preferable golf club has a low initial backspin rate and a great launch angle. The skilled in the art have also recognized that a golf club having a smaller height of a center of gravity tends to have a lower initial backspin rate and a greater launch angle. A light crown is employed and most of weights are distributed close to the sole so that a golf club having a low center of gravity can be obtained.
Although the crown is thinned in order to lower the center of gravity, the rigidity and strength of the crown becomes insufficient due to the thinning. The insufficient rigidity excessively deforms a head at time of an impact. The excessive deformation deteriorates the resilience performance due to energy loss and changes the hitting sound. In particular, a large-sized head is formed from each member that is originally thinned. Therefore, further thinning causes the insufficiency of the rigidity to be remarkable.
Japanese Patent Publication No. 7-98076 has disclosed a golf club head in which a grain size in the metallographic structure of a crown is reduced. Although the strength of the crown is enhanced by the adjustment of the metallographic structure, an effect thereof has limitations. Only the adjustment of the metallographic structure cannot sufficiently meet a demand for reducing the weight of the crown.
Japanese Laid-Open Patent Publication No. 11-155981 has disclosed a head in which a dent is formed on a crown. Although the rigidity of the crown is enhanced by the dent, an effect thereof has limitations. In addition, the dent remarkably deteriorates the appearance of the crown so that a sense of incompatibility is given to a golf player. Since golf is a mental sport, things to give the sense of incompatibility are kept at a distance by the golf player.
Japanese Utility Model Application No. 3063897 has disclosed a head in which a large number of ribs are formed on the crown. Although the rigidity of the crown is enhanced by the ribs, an effect thereof has limitations. In addition, a reduction in the weight of the crown reinforced by the ribs also has limitations.